Interpretive Summary: Disease causing microorganisms are commonly found in animal manure. Most transport experiments with microorganisms, however, have been conducted in the absence of manure particles. The potential implications of manure particles on microbe transport are not yet known. This study examined the transport behavior of manure particles and a type of disease causing microorganism, cysts of Giardia, in several sands. Results indicated that manure particles and Giardia were retained in the small pore spaces of the sand. Over time these sites were filled with manure particles, and the migration potential of larger sized particles and microbes was enhanced. Concentrations of Giardia that passed through the sands were therefore higher in the presence of manure particles. This result indicates that transport studies conducted in the absence of manure particles may underestimate transport potential of microbes in manure-contaminated environments.

Technical Abstract:
Column experiments were conducted to elucidate the transport behavior of cysts of Giardia and manure suspensions through several aquifer sands. Decreasing the median grain size of the sand resulted in lower peak effluent concentrations and increased deposition of the Giardia and manure particles in the sand near the column inlet. The effluent concentration curves for the manure suspensions also exhibited asymmetric shapes that tended to include larger particle sizes as the manure suspension was continuously added. This was more pronounced in finer sands. Simulations of the transport of Giardia and manure particles using a simple and flexible power law model for the solid-water mass exchange term provided a satisfactory description of the effluent and spatial distribution data. The cumulative size distribution (CSD) of manure particles in the suspension initially and after passage through the packed columns was used to identify the mechanisms that were controlling the deposition of manure particles and Giardia. CSD data indicated that manure particles were completely removed at early times by mechanical filtration and/or straining when the ratio of the particle to the median grain diameter was greater than 0.003-0.017, with increasing straining occurring in finer sands. However, the CSD changed with increasing time due to deposition-induced filling of straining sites. The Giardia transport was controlled by straining and/or mechanical filtration. For a given sand, higher effluent concentrations of Giardia were observed in the presence than in the absence of manure suspension. The relative increase of Giardia in the effluent concentrations varied from 75% to 172%. Hence, pathogen transport studies conducted in the absence of manure suspension may underestimate transport potential in manure-contaminated environments.